I don't know about you, but something I've always wanted was a small practice amp - battery power capable - into which I could mix an iPod and my guitar, and output to headphones or a small speaker. That way, I could listen to and work on learning tunes without bothering anybody nearby OR take off the headphones and just have a nice little amp. Neat, self contained, rugged.

I don't know about you, but something I've always wanted was a small practice amp - battery power capable - into which I could mix an iPod and my guitar, and output to headphones or a small speaker. That way, I could listen to and work on learning tunes without bothering anybody nearby OR take off the headphones and just have a nice little amp. Neat, self contained, rugged.

I would have used opamps no doubt about that. The will still put some of your EE design juices together. And the sound will be better. However if you still want to follow the transistor path. Take a look here http://forum.allaboutcircuits.com/showthread.php?t=73412
In some of the designs. You may not be able to find the transistors used. But we may help you in finding equivalent models

Didn't you do the simple calculation for the peak current?
1W into 4 ohms is (the root of 1 x 4, x 1.414=) 2.83V peak. Then the peak current is (2.83V/4 ohms=) 708mA which will blow up the tiny 2N3904 transistors.

Amplifiers usually use a class-AB output stage that has a pair of complementary power transistors as emitter-followers, driven by a voltage-amplifying stage.

Your design has a class-A output transistor as a heater instead of as an amplifier.

A little LM386 amplifier with a 9V supply has an output of only 0.4W at clipping into 4 ohms and the IC gets very hot.

I built the attached amp to play my MP3 player through a speaker, and I use it regularly into a 4Ω speaker. The audio quality is good enough for my purpose if I limit the drive to keep the output at 3.6 Vpp or less. The volume is loud enough for my somewhat diminished hearing. The IC is not even warm to the touch.

There are certainly better amplifier IC's than the LM386, but it works for some purposes. There are lots of videos on youtube of people using a guitar amp called a "ruby amp" that uses a MPF102 and an LM386.

An LM386 amplifier with a 9V regulated supply and a 4 ohm speaker has an output (with horrible distortion) of 0.4W and it heats with 0.9W if it has lots of sustain for almost continuous full output power.

An LM386 works best with an 8 ohm speaker. Its output with a 9V regulated supply is 0.56W when clipping a little and is 0.45W with low distortion. It heats with 0.53W if it has lots of sustain for almost continuous full output power.

A little 9V "transistor" battery will croak and die quickly when powering the amp driving a 4 ohm speaker when the peak current is 450mA.
Which gets hotter, the little amp or the little battery?

I don't have a graph of a little 9V battery being shorted with 450mA but here is 400mA:

An LM386 amplifier with a 9V regulated supply and a 4 ohm speaker has an output (with horrible distortion) of 0.4W and it heats with 0.9W if it has lots of sustain for almost continuous full output power.

An LM386 works best with an 8 ohm speaker. Its output with a 9V regulated supply is 0.56W when clipping a little and is 0.45W with low distortion. It heats with 0.53W if it has lots of sustain for almost continuous full output power.

A little 9V "transistor" battery will croak and die quickly when powering the amp driving a 4 ohm speaker when the peak current is 450mA.
Which gets hotter, the little amp or the little battery?

I don't have a graph of a little 9V battery being shorted with 450mA but here is 400mA:

Click to expand...

I respect your opinion and don't have the expertise (or the inclination) to argue with your data. I am simply stating my real world experience after having built the amplifier and used it on a regular basis for the last several months. Neither the amplifier nor the battery has ever gotten hot, and I have used only two batteries during the entire time period; the second one is still in use.

A JFET and an LM386 running off a 9V battery is worth trying.
I would go with an 8Ω speaker instead of 4Ω.
I'm going to breadboard this and report back.

Click to expand...

I just bought a TLV272IP (dual rail-to-rail op-amp) for my parts pile. Could that be used in place of the JFET stage?

A lot of those cheap "PC" speakers use a LM386. I have a bunch laying around and I've often thought of mounting a battery holder and pre-amp inside one. Instant practice amp. I could even leave the existing transformer in place for AC operation.

I have something similar so far. I will draw up the circuit after I get a chance to test it with a guitar.
I am using a MPF102 in common-source configuration.
(560Ω on source bypassed with 10μF. 1k8Ω on drain.)
20mV input gives 2V into 10Ω load (have not tried speaker as yet).